Hi, On my workbench, I have three broken Nikon FM's that I am cobbling together into one working unit. In replacing the focus screen (which was scratched) I had to remove the metal cage that holds the screen,a few shims and a leaf spring to hold it all together. But I didn't realize while taking it apart that there were little shim washers under the four screws that hold this cage to the body. The shim washers adjust the height of the cage that holds the focus screen, and therefore, affect focus calibration. I think I got the right shims reinstalled, but I'm not sure. So I wanted to check the focus calibration. Am I doing this right? Method: Camera on tripod, 135mm F2 mounted and pointing down 45 degrees towards a focus calibration chart. A bright desk light shines on the chart. I taped a piece of plain ground glass (no fresnel) to the film plane on the back of the camera. I then focused on the center of the chart, as accurately as possible. Then, on bulb mode, open the shutter and look at the image on the ground glass with a loupe. This showed the Nikon back-focusing about 12mm. I tried another camera, a Minolta SRT-102, and it showed a 10mm front focus using the same technique. Should I be putting a shim between the ground glass and the film plane to account for the thickness of the film? Is this method sound? Should I be adjusting for zero?